Variable resistor



Dec. 1, 1964 J. BORDEAUX 9,

I VARIABLE RESISTOR Filed Nov. 6, 1961 2 Sheets-Sheet l FIG.I

ATTORNEYS Dec. 1, 1964 J. BORDEAUX 3,159,805

VARIABLE RESISTOR Filed Nov. 6, 1961 2 Sheets-Sheet 2 FIG. 3

INVEN7OR.

JEAN BORDEAUX A TTORNE YS.

United States Patent 0 3,159,865 VARIABLE RESSIR Jean Bordeaux, Santa Ana, Cali., assigner to Robertshaw Controis Company, a corporation cf Delaware Filed Nov. 6, 1961, Ses. No. 159,282 17 Claims. (Cl. 338-106) This invention relates to variable electrical resistors such as carbon pile resistors in which the pressure acting on a pile or series of resistance elements is varied to change the resistance of the series. Among the disadvantages of present pile type resistors is that With re peated changes in pressure on the resistance elements the particles of material soon become caked or permanently compacted and Will not return to their original condition of compaction when the*pressure is removed. Another problem encountered With pile type resistors is that it is difiicult to obtain a uniform contact pressure between the adjacent elements in the series Wnich is required in order to obtain a linear resistance variation in response to pressure changes on the series.

It is, therefore, an object of this invention to obtain a linear variation in electrical resistance in response to temperature changes.

A more specifie object is to uniformly vary the contact pressure between adjacent elernents of a series of resistance elements to obtain a linear variation in the resistance of the series in response to changes in pressure on the series.

Another object is to obtain a uniform contact pressure between adjacent resistance elements of a series as the pressure on the series is varied.

A further object lies in the provision of a resistor in which variations in temperature cause a variation in the pressure aeting on a series of resistance elements and in which the initial pressure on the series can be adjusted in accordance With the temperature range to be encountered.

In the attainment of these objects a series of resistance elements, such as carbon blocks or dises, are supported between abutments which are relatively movable to increase and decrease the contact pressure between the adjacent elements. Interposed between each adjacent pair of eiernents is a spring which tends to bias the elements apart. With each of the springs eXerting substantially equal biasing forces, the contact pressure between the adjacent elements throughout the series Will be substantially uniform. As the pressure acting on the ends of the series of elements varies, the elements move toward and away from each other to vary the resistance between each pair of elements.

Tire device of this invention is partieularly useful in controls such as electric oven eontrols, in which high ternperatures are encountered and where it is desired to vary the fiow of current in accordanee With ternperature variations. For this purpose, it is preferred that the resistance be carbon blocks or dises as carbon is capable of handling relatively high currents. Many spring materials lose their biasing eiect at high temperatures, and the material selected for the biasing means shonld be capable et Withstanding approximately 1200 F. without losing its resiliency. One suitable material is a high nickelchromiumiron alloy sold under the tradename inconel-X by the International Nickel Company.

In one embodiment of the invention a pair of cylindrical members are threadedly connected to form a cylindrical casing having a pair of end walls. Axially dis posed in the casing is a cylindrical insulating sleeve, and a series of resistance elements eonsisting of carbon dises each formed With an annular flange is disposed within the insulating sleeve between the end walis of the casing. 7

Concentricaily mounted on each of the carbon dises is 3,159,805 Patented Dec. 1, 1964 a ring of insulating material, and a bowed-spring Washer has an inner peripheral edge concentrically mounted on tire insulating ring With its outer peripheral edge abutting the back of the adjacent disc. Insulating dises are disposed between the ends of the series and the end walls of the casing. in order to connect the series of elements to an electrical circuit, the dise at each end of the series is provided With an electrical lead connection. Variations in ambient temperature on the casing cause the end walls to move relative to each other thereby increasing and decreasing the pressure on the resistance elements. The bowed-spring washers maintain a uniform contact pressure between adjacent elements throughout the series. By relative rotation of the pair of cylindrical casing members, the pressure on the resistance elements can be initially adjusted in accordance With the range of temperatures to be encountered.

in a second embodiment of the invention, a series of. resistance elements, such as carbon dises, is mounted on a support having a pair of spaced Walls joined by connecting rods. The eiements are disposed between a stationary abutment and a relatively movable abutment whieh is slidably mounted on the connecting rods. Formed in each face of the resistance elements in a recess for receiving a cup-shaped insulating mernber, preferably of ceramic material. Mounted between the movable abutment and the adjacent end wall of the support member is a capsule which expands and contracts in response to temperature variations. Upon an increase in ambient temperature, the capsule expands causing the movable abutment to rnove toward the stationary abutment and increase the pressure on the resistance elements. Accordingly, the resistance varies inversely With temperature changes acting on the capsule. Mounted between each pair of elements is a helical spring with its ends received in the opposed cup-shaped insulating members to exert a biasing force tending to separate the elements. An adjusting screw is provided for preadjusting the initial pressure on the resistance elements in accordance With the range of temperatures to be encountered.

The objecte and advantages of the invention will beeome apparent from the following description taken in connection With the accompanying drawings in whieh:

FG. 1 is a sectional view of a first embodiment of the invention;

FIG. 2 is an exploded perspective view of the spring washer, insulating ring and resistance element of the device of FG. 1;

FG. 3 is a partial sectional view of a second embodi ment of the invention; and

FIG. 4 is an exploded perspective view of the embodi ment shown in FIG. 3.

in the device of FIG. 1, the series of resistance clements is mounted in a support or casing 19 which consists of a pair of cylindrical casing members 12 and 14 joined together by connecting threads l6. Casing members 12 and 14 have end Walls 18 and 20, respectively. Openings 22 and 24 having inturned fianges 26 and 28 are formed in end Walls 18 and 20. For temperature ranges encountered in oven controls, the casing material is preferably stainless steel.

Axially dispesed in the casing 10 is a cylindrical insulating sleeve 30 which is preferably of ceramic material. Tire ends of the insulating sleeve 36 are spaced from the end walls 13 and 20 to provide for expansion and contraction of the easing member 14). Mounted in each opening 22 and 24 is an insulating disc or block 32, also pref erably of ceramic material, having annular flanges 36.

0 In order to connect the resistance to an electrical circuit,

resistance elements 33 in the form of a disc having an annular flange 49 (sec FIG. 2). Resistance elernents 38 may be carbon discs or blocks.

Concentrically mounted on each of the resistance elements 38 is an insulating ring 42 having an annular fiange 44 (sec FIG. 2). Insulating ring 42 also may be of ceramic material. The flanged portion 44 of ring 4?; abats the fiange 49 of resistance element 33. Biasing means in the form of a bowed-spring washer 46 is interposed between each pair of elements 38, and as shown in FG. 2, the washer 46 has an inner peripheral edge and outer peripheral edge Spring Washer 46 is mounted between the elements in such a manner that its inner peripheral edge 48 is concentrically received on insulating ring 42 at the junction between flange 44 and the main portion of insulating ring 42. The enter peripheral edge 56 abuts against the back of the adjacent resistance element 38. Recesses are formed in the resistance elements at the opposite ends of the series for receiving a conventional fastener for connection With electrical lead wires 52 As the end walls or abutments 38 and 26 move relative to each other, the contact pressure between the resistance elements 38 increases and decreases Which varies the electrical resistance of the series. The contact pressure between adjacent elements is maintained uniform throughout the series by the spring washers 46. Relative rotation of casing members 12 and 14 initially adjusts the pressure on the series of elements for the temperature range to be encountered, and as the casing elongates and contracts With ambient temperature Variations, the1 contact pressure between the elements varies accordmg y.

A second embodiment of the invention is illustrated in FIGS. 3 and 4 and comprises a support 60 Which consists of end wall members 62 and 64 joined by a pair of connecting rods 66. Connecting rods 66 should be of a material having a relatively low coefficient of thermal expansion, such as Invar, so that the relative positions of wall members 62 and 64 will remain fixed With temporaturc variations. To insulate connecting rods 66, cylindrical insulating members 68 are mounted on rod 66 which are preferably of ceramic material.

Slidably mounted on connecting rod 66 and insulating members 68 is a movable abutment 7@ of ceramic insulating material. Movable abutment 70 bas a pair of diametrically opposed notches 72 formed at its periphery for receiving the insulated rods 66.

Mounted between the movable abutment 70 and wall mcmber 64 is an expansible capsule 90 Which is filled with fluid that expands and contracts in response to temperature variations. Expansion and contraction of capsule 90 causes movable abutment 70 to slide along rode 66 toward and away from wall member 64. Formed in the face of abutment member 76 opposite capsule 96 is a recess 71 for receiving a plate 86 of conductive material, such as brass, which has an electrical lead Wire 88 connected thereto.

Mounted adjacent wall member 62 is a stationary abatment 74. Threadedly mounted in wall member 62 is an ad;ustmg screvv 76, the end of Which engagea stationary abutment 74. Stationary abutment 74 is also preferably of ceramic material and has a recess 75 formed in its face opposite wall member 62 for receiving a second plate 86 of conductive material, also having a lead Wire 88 connected thereto.

Mounted between stationary abutment 74 and movable abutment 70 is a series of resistance elements 78 such as carbon dises or blocks. Formed in each face of resistance elements 78 is a recess 86 for receiving the cup-shaped insulating member 32 of ceramic material. The outer faces of discs 79 at each end of the series is provided With an enlarged recess 86 for receiving plate member 86.

Mounted between each pair of elements 78 is a helical spring 84 having its opposite ends mounted in the opposed insulating members 82 of the adjacent elements, Spring 84- exerts a biasing force on the elements tending to separate them.

In operation stationary abutment 74 is initially positioned by rotation et adjusting screw 76 to place the resistance elements 78 under compression between abutmente 74 and 70. The initial position of abutment 74 is determined by the range of temperatures to be encountered. As the ambient temperature on capsule 96 increaees, it expands and exerts a force on movable abutment 76 tending to move it toward abutment 74 increasing the contact pressure between the elemcnts 78. The contact pressure is maintained uniform throughout the series by the springs 34 which exert forces in opposition to the compressive forces exerted by abutments 76 and 74. Therefore, as the temperature increases causing expansien of capsule 9%), the contact pressure between the resistance elements increases and the resistance of the series thereby varies inversely with the temperature. As the temperature decreases, the pressure between the elements is decreased, and the spring 84 tending to separate the elements prevents the material of the resistance elements 76 from becoming permanently compacted. Since the contact pressure is maintained uniform between the adjacent resistance elements, a linear relationship is obtained between the pressure exerted by capsule and resistance. If capsule 90 is completely filled with a liquid that expands and contracts With temperature variations, the resistance will Vary linearly With temperature.

While the above embodiments of the invention have been described for purposes of illustration, it will be understood that various modifications can be made by thcse sl illed in the art without departing from the scope of the invention as defined by the appended claims.

What I claim is:

1. A variable resistor comprising in combination support means having a pair of members movable relative to each other in response to temperature variations, a series of solid resistance elements mounted between said members whereby said relative movement increases and decreases the pressure on said elements to vary the resistance of said series in response to temperature variations, insulating means mounted on each of said elements, resilient means between each adjacent pair of said elements engaging said insulating means and tending to sepa rate said elements to maintain a uniform contact pressure between the adjacent elements throughout said series, means for connecting said series With an electrical circuit, and means for adjusting the initial relative positions of said movable members in accordance with the temperaturc range to be encountered.

2. A variable resistor comprising in combination a series of resistance elements mounted between members movable relative to each other to increase and decrease the pressure on-said elements to thereby vary the resistance of said series, and a boWed-spring Washer interposed between each adjacent pair of said elements tend ing to separate said elements and having an inner peripheral edge concentrically mounted on one element of said pair and an outer peripheral edge engaging the other element of said pair to maintain a uniform contact pressure between the adjacent elements throughout said series, and insulating means interposed between the bowed spring washer and at least one of each adjacent pair of resistance elements providing a path for electrical current through the resistance elements only.

3. A variable resistor comprising in combination a series of resistance elements mounted between members movable relative to each other in response to temperaturc variations to increase and decrease the resistance of said series in response to temperature Variations, each of said elements having a fiange, an insulating member r1ounted on each of said elements in engagement With said fiange, and spring means between each pair of elements having one and mounted on said insulating member and the other end in engagement With the adjacent element tending to bias said elements apart to maintain a substantially uniform contact pressure between adjacent elements throughout said series whereby the resistance of said series varies linearly with ambient temperature changes.

4. The construction defined in claim 3 further including apertured insulating blocks mounted between said relatively movable members and the ends of said series, electrical lead wires connected to the resistance elements at each end of said series and extending through said apertured insulating blocks, and means for adjusting the initial relative positions of said relatively movable members.

5. A variable resistor comprising in combination a series of resistance elements mounted between members movable relative to each other to increase and decrease the pressure on said series thereby varying the resistance thereof, each of said elements comprising a dise having an annular fiange, an insulatin ring concentrically mounted on each of said elements in engagement With said annular flange, and a bowed-spring washer interposed between each pair of said elements tending to separate said elements and having an inner peripheral edge concentrically mounted on said insulating ring and an outer peripheral edge abutting the adjacent element to maintain a uniform contact pressure between adjacent elements throughout said series.

6. A variable resistor comprisng in combination a casing having end walls, an insulating sleeve in said casing, a series of resistance elements axially disposed in said sleeve between said end walls whereby relative movement of said end walls due to temperature variations will increase and decrease the contact pressure between adjacent elements in said series, an insulating member on each of said elements, and resilient means for each pair of elements engaging said insulating member and the adjacent element and tending to separate said elements for maintaining a uniforrn resistance between adjacent elements throughout said series whereby the resistance of said series of elements will vary in accordance With temperature changes.

7. A variable resistor comprising in combination a pair of cylindrical members threadedly connected to form a casing whereby relative rotation of said cylindrical members causes said casing t0 elongate and contract, said casing having oppositely disposed end walls, a series of resistance elements axially disposed in said casing between said end walls whereby relative movement of said end walls in response to temperature variations increases and decreases the resistance of said series, said elements each comprising a dise having an annular flange, an insulating ring concentrically mounted on each of said elements in engagement With said annular flange, and a bowed-spring Washer between each pair of elements having an inner peripheral edge in engagement With said insulating ring and an outer peripheral edge engaging the adjacent element of said pair and tending to separate said elements to maintain a uniform resistance between said elements throughout said series whereby the resistance of said series varies uniformly With temperature changes.

8. T he construction defined in claim 7 further including a cylindrical insulating sleeve axially disposed in said casing, said series of resistance elements being disposed Within said insulating sleeve, apertured insulating dises mounted between said end walls and the ends of said series, and electrical lead wires extending through said apertured insulating dises and connected with the resistance element at each end of the series for connection with an electrical circuit.

9. A variable resistor comprising in combination a series of solid resistance elements mounted between members movable relative to each other to increase and decrease the pressure on said elements to vary the resistance of said series, each of said elements having a recess formed in each face thereof, a cup-shaped insulating member mounted in eachrecess, and a spring interposed between each pair of said elements tending t0 separate said elements and having its ends received in the opposed cupshaped insulating members to maintain a uniform contact pressure and resistance between the adjacent elements throughout said series.

10. The construction defined in claim 9 including temperature responsive means operatively connected With one of said relatively movable members to cause said members to move toward each other upon an increase in ambient temperature whereby the resistance of said series varies inversely With temperature changes.

11. A variable resistor comprising in combination support means including a pair of end walls joined by connecting rods, a relatively fixed abutment mounted adjacent one of said end walls and a movable abutment mounted adjacent the other of said end walls, a series of solid resistance elements mounted between said abutments, means interposed between said elements tending to bias the adjacent elements cf said series apart, means mounted between said movable abutments and said other end wall expansible in response to increases in ambient temperature t0 urge said movable abutment toward said fixed abutment and increasing the pressure on said resistance elements whereby the resistance of said series varies inversely With temperature changes.

12. A variable resistor comprising in combination support means including a pair of spaced end walls joined by connecting rods, a relatively fixed abutment mounted adjacent one of said end walls and a movable abutment mounted adjacent the other end wall, a series of resistance elements mounted between said abutments whereby relative movement of said abutments increases and decreases the pressure on said elements to vary the resistance of said series, resilient means between each pair of said elements tending to bias the elements apart to maintain a uniform contact pressure and resistance between adjacent elements throughont said series, and means between said other wall and said movable abutment expansible upon increases in ambient temperature t0 cause relative movement of said abutments whereby the resistance of said series varies inversely With temperature.

13. The construction defined in claim 12 further including means for adjusting the position of said relatively fixed abutment in accordance With the temperature range to be encountered.

14. The construction defined in claim 12 wherein each of said resistance elements has a recess formed in each face therecf, and including a cup-shaped insulating member received in said recess, and said resilient means comprises a spring having its ends mounted in the opposed insulating members.

15. The construction defined in claim 12 further including a series of cylindrical insulating elements mounted on said connecting rods.

16. A variable resistor comprisng in combination, support means including spaced end walls joined by insulated connecting rods, a pair or: relatively rnovable abutments of insulating material mounted on said support means, a series of resistance elements mounted between said abatments whereby relative movement of said abutrnents increases and decreases the pressure on said elements t0 vary the resistance of said series, recesses formed in the opposed faces of said elements, insulating means in said recesses, resilient means between each pair of elements tending to separate said elements having its ends abutting the opposed insulating means to maintain a uniform contact pressure and resistance between adjacent elements throughcut said series, a plate of conductive material mounted between each of said abutments and the adjacent resistance element, means on said plates for connecting said series of resistance elements in an electrical circuit, and means expansible in response to temperature changes operatively connected With one of said abutments for causing relative movement of said abutments whereby 7 the resistance of said series varies in response to temperapressure between the elements throughout the series, ture changes. and means for moving said abutments relative to each 17. A variable resistor comprising other to increase decrease the contact pressure a i of 5paced abutments between adjacent peurs of the elements t0 vary the resistance of the series.

a series of solid resistance elements mounted between abtmentS Reiesences Cted in the file of this patent a cyhndncal Wall defined on each resrstance element,

an insulating member mounted on each resistance ele- UNTED STATES PATENTS ment concentrically engaging the cylindrical wall, 10 1,521,213 Baruch Dec. 30, 1924 a sprng interposed between each adjacent pair of the 1,984,947 Schweibold Dec. 18, 1934 resistance elements and concentrically engaged with 98,679 Thompson Apr. 16, 1946 the insulating member on at least one of said pair FOREIGN PATENTS of resistance elements tending t0 resiliently separate said elements axially to maintain a uniform contact 15 220361 Great Bmam 1924 

1. A VARIABLE RESISTOR COMPRISING IN COMBINATION SUPPORT MEANS HAVING A PAIR OF MEMBERS MOVABLE RELATIVE TO EACH OTHER IN RESPONSE TO TEMPERATURE VARIATIONS, A SERIES OF SOLID RESISTANCE ELEMENTS MOUNTED BETWEEN SAID MEMBERS WHEREBY SAID RELATIVE MOVEMENT INCREASES AND DECREASES THE PRESSURE ON SAID ELEMENTS TO VARY THE RESISTANCE OF SAID SERIES IN RESPONSE TO TEMPERATURE VARIATIONS, INSULATING MEANS MOUNTED ON EACH OF SAID ELEMENTS, RESILIENT MEANS BETWEEN EACH ADJACENT PAIR OF SAID ELEMENTS ENGAGING SAID INSULATING MEANS AND TENDING TO SEPARATE SAID ELEMENTS TO MAINTAIN A UNIFORM CONTACT PRESSURE BETWEEN THE ADJACENT ELEMENTS THROUGHOUT SAID SERIES, MEANS FOR CONNECTING SAID SERIES WITH AN ELECTRICAL CIRCUIT, AND MEANS FOR ADJUSTING THE INITIAL RELATIVE POSITIONS OF SAID MOVABLE MEMBERS IN ACCORDANCE WITH THE TEMPERATURE RANGE TO BE ENCOUNTERED. 